Recently, by combining a light-emitting diode (LED) chip for radiating blue or ultraviolet rays based on a gallium nitride semiconductor with a fluorescent material of various types, a light-emitting device which emits a light of a color or colors, including white, different from that of the emitting light of the LED chip has been developed. The light-emitting device has advantages such as small size, light weight and low dissipating power, and it is used widely for a light source for display, a light source in substitution of a small electric bulb, a light source for a liquid crystal display, and the like. When such a light-emitting device is used for a light source for display, a light source for liquid crystal panel or the like, the brightness per chip is low and insufficient. Then an LED package is provided to have an LED chip mounted and sealed on a mount base having electrically conducting portions to be connected to an external circuit, and a required number of LED packages are generally mounted on a print circuit board. For example, Japanese Patent laid open Publication 2001-134558 discloses a light-emitting device wherein a metallic bloc having a light-emitting diode chip in a recess thereof is mounted on a metallic flat plate.
In order to provide a high light intensity, an injection current for an LED chip may be increased. Because an LED chip available now has efficiency as small as 10%, a large part of input electric energy is converted to heat, so that the amount of heat increases with increasing current. It is known that characteristics such as life and efficiency of LED chip are deteriorated when the temperature rises due to the generated heat. Because the print circuit board for mounting the LED package is generally made of a resin such as polyimide or epoxy resin having low thermal conductivity, the generated heat cannot be radiated efficiently from the LED package.
FIG. 1 shows an example of a prior art light-emitting device 99 which efficiently transfers the heat generated in LED packages (for example, Japanese Patent laid open Publication 2002-162626). Each of the LED packages 90 having a pair of external terminals 95 of so-called plane mount type is mounted on a film base 92 which is a print circuit board made of polyimide. Lands 93 of electrically conducting patterns are formed on a top plane of the film base 92, while the back plane thereof is bonded with an adhesive to a supporting frame 91 made of a metal. The electrodes 95 of the LED packages 90 are connected to the lands 93. Further, holes are formed vertically through the film base 92 and the frame 91 at regions below the LED packages 90, and an adhesive filler 94 having high thermal conductivity is filled into the holes to the back of the LED packages 90. A part of the heat generated by the LED chips conducts via the lands 93 to the film base 92 and further to the frame 91 to be radiated therefrom. Further, a large part of the heat generated in the LED package conducts directly through the adhesive filler to the film base 92 and further to the frame 91 to be radiated therefrom.
However, the above-mentioned heat transfer structure of a light emitting device has the following problems. The adhesive filler made mainly of silicone resin and having high thermal conductivity is used to conduct the heat generated in the LED chip, but it has a smaller thermal conductivity than a material such as a metal or a ceramic. Further, at least a step for filling the filler into the holes for forming heat transfer paths is needed in a packaging process thereof, besides a step for mounting the LED packages 90 to the lands 93. Further, the filling step is troublesome.